Battery Capacity Calculator
A battery capacity calculator helps you estimate how many amp-hours you need for a specific load, voltage, and runtime. This is one of the most useful sizing tools for lithium batteries because it converts energy demand into a practical battery size you can actually shop for or design around.
This page calculates required watt-hours, required usable watt-hours, required amp-hours, and recommended battery size using runtime, voltage, depth of discharge, and efficiency. It works well for solar backup, RV systems, marine loads, off-grid setups, and general lithium battery planning.
Short answer: Start with load in watts multiplied by runtime in hours to get watt-hours, then divide by system voltage and adjust for usable depth of discharge and efficiency to estimate the battery capacity you need in amp-hours.
Required Battery Ah Calculator
Estimate required battery capacity from load, runtime, voltage, battery chemistry, usable DoD, and system efficiency
Features
Any Load
Calculate required amp-hours for almost any electrical load, from small electronics to larger inverters and backup systems.
Desired Runtime
Size the battery around how many hours you actually want the system to run, not only around power draw.
Common Voltages
Choose common 12V, 24V, and 48V battery systems or enter a custom voltage for special setups.
DoD and Efficiency
Adjust for usable depth of discharge and system efficiency so the required battery size is more realistic than a simple ideal calculation.
Capacity Formula
The starting point is the basic energy formula:
Required Wh = Load (W) × Runtime (hours)
The simple amp-hour version is:
Required Ah = Wh ÷ Voltage ÷ DoD
When you also want to account for losses, a more practical version is:
Required Ah = Wh ÷ Voltage ÷ DoD ÷ Efficiency
Here, DoD and efficiency should be used as decimals in the actual math, such as 0.8 for 80% and 0.95 for 95%. That gives you a more realistic required battery size than using ideal energy only.
Battery Chemistry Assumptions
| Battery Type | Typical Usable DoD | Notes |
|---|---|---|
| LiFePO4 | 80% | Common conservative planning value for routine use |
| Li-ion | 80%–85% | Common planning range depending on battery design |
These are planning assumptions, not absolute rules. Some manufacturers allow deeper discharge, but repeated deep cycling can affect longevity, so conservative sizing is usually safer for daily-use systems.
Worked Example
Your example starts with a 500W load running for 4 hours, which equals 2000Wh of required energy. Using 12V and 80% usable DoD gives 2000 ÷ 12 ÷ 0.8 = 208.3Ah, which matches the standard battery capacity calculation you provided.
If you also include 95% system efficiency, the required size becomes slightly larger because the system has to cover conversion and wiring losses. That is why practical battery sizing often ends up a bit higher than the simple formula result.
Practical warning: Real battery systems should include reserve margin. If your calculated answer is 208Ah, many real designs would round up to the next standard size or add extra capacity for aging, cold weather, or surge demand.
Solar and Backup Use
Solar Systems
For solar sizing, start with your daily energy use in Wh, then convert that into required battery Ah at your chosen battery voltage and usable DoD.
Backup Days
If you want multiple days of backup, multiply your daily energy need by the number of backup days before converting to amp-hours.
Inverter Loads
When powering AC devices through an inverter, include efficiency losses so your battery estimate is not too small.
Battery Selection
Once you know the required Ah, compare that figure with standard battery sizes such as 100Ah, 200Ah, and 300Ah packs.
Practical rule: Size for real usable capacity, not rated nameplate capacity alone. Depth of discharge and efficiency make a big difference to the final battery size.